Factors affecting (H ion) concentrations in plaques are being determined in horizontal studies using plaques accumulated on carious and on sound surfaces of children with varying past and present caries experience. Components showing large differences between the two types of plaques will be assayed in a longitudinal study using children where families have a low probability of moving out of the area. Correlations are to be made between caries experience, plaque chemical components (e.g. individual fermentation product acids), and the dietary history of the children studied.